Why do you want to use a BitmapData? Not sure what you're after, but after reading a couple of your questions it seems you're a kind of fighting against how flash works. I think you'll make things much easier for yourself if you use what's available already. BitmapData objects are meant mainly to manipulate pixels and don't expose methods for drawing shapes. A Graphics object (available through Sprite, Shape, etc) on the other hand, allows you to draw vector shapes easily.
So, the best approach for this, I guess, would be using the drawing API to do what you want and then, if needed, convert the graphic to a BitmapData.